This invention relates broadly to the recovery of solid wastes from the effluent stream generated in the manufacture of melamine and, more particularly, to the preparation of resin compositions from the solid waste from the melamine industry, and to the use of such resin compositions as superplasticizers for admixture with concrete.
A major problem with most processes in the petrochemical industry is the contamination of the effluent streams with various chemicals, including unrecovered amounts of the primary product of the process. This is particularly important in the case of the melamine industry.
Melamine is manufactured on a commercial scale by converting urea to melamine in several stages, including a crystallization stage which purifies the melamine to the required specifications. The mother liquor following the crystallization stage is normally stripped of ammonia and concentrated to a solids content of about 1.5-5% by weight. This final effluent stream, which is usually disposed of as waste water, contains various proportions of melamine, oxyaminotriazines, cyanuric acid, melam, melan, melon, biuret, triuret, and other higher polycondensates of urea and melamine. These various components, the percentages of which vary depending on process conditions, are considered to be the major contributors to the pollution problems of melamine manufacturing plants. In addition to the pollution problems created by these waste solids, the actual tonnage of melamine lost in these waste solids is rather substantial. Accordingly, it would be most desirable from both an economic and ecological standpoint to recover the waste melamine and by-products in a commercially usable form.
The problem of recovering these waste materials has been dealt with by various melamine manufacturers and research organizations during recent years. For example, it has been demonstrated that the solid content in the effluent streams can be reduced by such varying techniques as biological hydrolysis, thermal hydrolysis, absorption on activated carbon, the production of cyanuric acid from wasted melamine, and the recovery of the various waste products by means of ion exchangers. While all of the above techniques can reduce somewhat the solid content of the waste effluent stream from a melamine process, none have been utilized commercially either because they are considered uneconomical or technically too complicated.
One such technique for reducing the melamine content in a waste effluent stream is taught in Kennedy, U.S. Pat. No. 3,496,176. In that patent, it is taught to adjust the pH of the mother liquor to 6-7, immediately after the ammonia stripper, by bubbling carbon dioxide in the effluent stream. In this manner, about 70-80% of the ammelide and ammeline, i.e., some of the by-products in the stream, are precipitated. The stream is then filtered to remove the precipitated solids and melamine is recrystallized from the filtrate. A similar procedure is disclosed in Dakli et al, U.S. Pat. No. 3,423,411 to precipitate oxyand oxyaminotriazines, except that in the recovery of melamine it was necessary to heat the filtrate and then pass the heated filtrate through a column of cationic exchange resins to remove other nitrogen-containing organic impurities and alkali ions. Still another technique for reducing the melamine content of the waste stream is discussed in Fujiyoshi, Japanese Pat. No. 50-26553. In that patent it is disclosed to blend the mother liquor from the melamine synthesis with cyanuric acid to precipitate melamine and melamine cyanurate.
Hoogendonk, U.S. Pat. No. 3,496,177, relates to a process for the purification of crude melamine by dissolving the crude melamine in water after adjusting the solution to a pH of 6-8, and then filtering at 105.degree. C. The filtrate containing melamine is then adjusted to a temperature of 50.degree. C. and crystallization of pure melamine is obtained. A similar procedure is disclosed in Rettler, German Pat. No. 1,162,379, except that after dissolving the crude melamine in water, the solution is treated with a strongly basic ion exchange resin at 70.degree.-75.degree. C. before the solution is filtered and the melamine recrystallized. Elmer et al, U.S. Pat. No. 2,863,869, discloses purifying melamine in an aqueous solution of sodium hydroxide (pH 11.5-11.9) at 129.degree.-141.degree. C., filtering the solution, and then cooling to recrystallize the melamine.
It should be apparent from a review of the above and other known techniques for dealing with the problem of melamine plant waste that efforts have been concentrated in the areas of:
(1) the recovery of the melamine waste from the waste effluent and its purification to obtain high purity melamine; PA1 (2) the recovery of the melamine waste from the waste effluent and its utilization as feed material for manufacturing cyanuric acid or melamine; and PA1 (3) the treatment of the waste effluent as a pollutant and subjecting the effluent to biological treatment. PA1 (1) treating the waste solids from the waste water stream with an aldehyde, suitably formaldehyde, for example, in the form of a formalin solution (30-40%) at a temperature on the order of from about 40.degree. C. to about 80.degree. C. after adjusting the pH to about 11-13; PA1 (2) treating the solution obtained in step (1) with a sulfonating agent, such as a metal sulfite, bisulfite or metabisulfite, at a temperature of from about 70.degree. C. to about 90.degree. C. for about 60 to about 180 minutes; PA1 (3) polymerizing the reactive components of the solution from step (2) at a pH of about 1-5 and at a temperature on the order of from about 30.degree. C. to about 80.degree. C. for about 10 to about 150 minutes; and PA1 (4) stabilizing the resulting resin solution by heating the same at a pH of from about 7-10 and a temperature of from about 50.degree. C. to about 100.degree. C. for about 30 to about 180 minutes.
Heretofore there have been no successful efforts to convert the solid wastes, i.e., the melamine together with the various by-products, that are contained in the waste effluent into a commercially usable product, thereby improving the economics of the melamine synthesis plant while simultaneously reducing the plant's waste disposal problems.